This invention relates generally to illumination systems for display devices and particularly to back illumination systems for color liquid crystal displays.
Color display devices are rapidly gaining popularity for visually displaying alphanumeric and graphic information. Additionally, industry is energetically pursuing the development of a color television display which is different from the standard color cathode ray tube. Liquid crystal display devices are at the forefront of these endeavors. The pixels of liquid crystal displays are composed of twisted nematic liquid crystal cells, and are arranged into a column and row matrix so that each pixel is associated with a particular row and column. The liquid crystal cells are individually addressed to actuate the cells to change their light transmitting characteristics, as needed by the particular display to be produced. Color displays are produced by associating a filter of a particular color with each liquid crystal cell so that each crystal cell will appear to transmit only one color of light. When the display is to be used for a color display, or color television, each pixel contains a triad of liquid crystal cells and each of the cells transmits either red, green or blue light. It is also possible to use pixels including four liquid crystal cells. In such a use the cells are arranged parallel to the axes of the screen and the pixels are square. The present invention is useful with all types of pixel configurations. The various other colors of the light spectrum are produced by properly combining the primary light colors. This technique is well known and is utilized in the CRT color picture tube art as well.
In liquid crystal displays, back illumination is frequently used to provide a bright, high-contrast display having a wide viewing angle; the back illumination must therefore be quite bright for the desired purposes.
In liquid crystal displays, the incoming light must be polarized and reasonably well collimated to make effective use of the liquid crystal electroptic effect and to produce a uniform display having acceptable contrast. A significant amount of light generated by the primary light source is usually lost in collimation. Also, a substantial percentage, typically more than 50%, of the incoming collimated light is lost to the polarizers. Additionally, because each of the liquid crystal cells within the pixels of the color display transmits only one color component of the light spectrum, approximately two thirds of the light transmitted by the polarizers is lost to the filters. Accordingly, 85%, or more, of the incoming collimated light to the display device is typically lost because of the inefficiencies of the polarizers and the filters. Another small percentage of light is also lost to other inefficiencies of the display device, such as the panel area devoted to conductors and active circuit elements required to actuate the crystals. For these reasons, there is a need for a high efficiency illumination system for a display device which eliminates a substantial portion of the light losses occasioned by the utilization of polarizers and color filters. The present invention fulfills these needs.